The present invention concerns products made of absorbent paper and more especially of cellulose wadding for sanitary or domestic use and aims to provide for a sheet made up of at least two plies one of which is embossed, intended, essentially, for the making of toilet paper but also for handkerchiefs, serviettes and general purpose wipes.
In the sanitary and domestic paper industry absorbent paper, generally creped and of low weight, is used for the making of these products, called: cellulose wadding or soft tissue. The most use is made of the structures capacity for stretching conferred, for example, by the creping in order to emboss the sheet, i.e. to deform it permanently in places and to obtain protuberances on one face corresponding to hollow zones on the other face.
In effect, the trend over the last few years concerning hygiene products has been to make them softer and smoother by working on their characteristics of thickness and strength, notably by embossing. The latter further allows improvement of the product""s visual attraction. The embossing process can be carried out either on paper with a high moisture content, i.e. in the wet end of the paper machine, or on paper with a low moisture content, i.e. during processing. The present invention concerns paper processed in the dry end.
The most widespread patterns of the embossing are made up of a repetition of basic protuberance with a small transverse section and a simple geometric form on a geometric basis. An example is described in U.S. Pat. No. 3,414,459 which focuses on a two- or three-ply sheet, i.e. made up of a plurality of single sheets, called plies, stuck or bonded together. The plies are embossed with a frequency of distribution and height of the protuberances adapted to the making of water absorbing products such as general purpose wipes, the number of elements going from 5 to 30 per cm2. Furthermore, the Applicant has developed, notably for toilet paper, some patterns whose number of elements is greater, going from 30 to 80 per cm2. In this case, because of their number, these elements form protuberances with a lesser height and necessarily have an elementary surface at the top which is also very small, less than 1 mm2. For these last embodiments an appearance which imitates that of a woven product is obtained. An example is described in Patent EP 426,548.
This type of embossing affects mainly the characteristics linked to the thickness of the sheet, on the one hand, and to its rigidity and strength, on the other. These patterns allow a good compromise to be reached between the desired improvements of the characteristics when the semi-finished product becomes the finished product, and the conditions of industrial operation; they allow, notably, the application of a sufficient intensity of embossing. However, they present limited visual appeal.
Other absorbent paper products are made up of two plies but made using a different method of joining them together. On the one hand, one ply or a group of two superposed plies is embossed and, on the other, a second ply is embossed by means of two distinct embossing devices. The first ply as well as the second are embossed according to the raised patterns formed by the protuberances. The pattern density remains necessarily low: less than 20 protuberances/cm2 for this type of product. In effect the two plies are joined together by means of a matchmaker cylinder in such a way that the distal surfaces of the protuberances of the two superposed plies are placed opposite the surfaces situated between two protuberances in the plane of the second ply. This technique is still called the xe2x80x9cnestedxe2x80x9d process. With a structure of this type it is sought to obtain products which are thick and are soft to the touch. However, especially for its application to toilet paper, softness is not optimal because of relatively coarse embossing. Furthermore, it is necessary to emboss heavily to obtain the considerable desired thickness.
In both methods of making a double sheet, both plies are embossed. If embossing gives thickness to each of the two plies it leads, however, to a noticeable reduction of their resistance to tearing. In effect, the mechanical work on the sheet is accompanied by a relaxation of the inter-fibre links in the embossed zones.
Thus, the prime objective of the invention is the production of a double sheet whose resistance to tearing is improved compared to an embossed double sheet of the same thickness.
Conversely, by increasing the pressure of the embossing the thickness of the sheet and/or the quality of the pattern marking of the embossing are increased but the process is limited by the necessity of not inducing too great a loss of resistance to tearing.
The second objective of the invention is the production of a double sheet whose thickness and/or visibility of the embossing pattern is improved, compared to an embossed double sheet in the prior art, while showing good resistance to tearing.
In accordance with the invention, a sheet satisfying one or the other of the two objectives set out above, made up of two plies of cellulose wadding each with a grammage of 10 to 40g/m2, one of which is embossed with relief patterns, consisting at least in part of discrete protuberances orientated towards the interior of the sheet, and the other is not embossed, is characterized in that the embossed ply has, on at least one part of its surface, at least 30 protuberances per cm2 whose surface area on the top is less than 1 mm2, preferably less than 0.7 mm2.
In effect, the Applicant has noted with surprise that a double sheet so characterized has the same thickness as a double sheet made with two plies embossed in the same way as the embossed ply of the invention and assembled in points/points position.
This property has only been observed for pattern densities at least equal to 30 spikes per cm2 on portions of the surface covering at least 30% of the total surface.
The surface of the tops of the protuberances is all the smaller the higher the number of these protuberances is locally. It is preferably inferior to 0.7 mm2 for 30 protuberances per cm2 and preferably inferior or equal to 0.4 mm2 for 50 protuberances per cm2.
By thickness, in the present technical field, is understood thickness measured on a stack of a certain number of sheets (12 plies, for example) on which a light pressure (e.g. 2 kPa) is exerted. It follows that the thickness takes into account a certain resistance to crushing of the structure. Thus without wanting to be bound by an explanation, conservation of the thickness of the structure of the invention results from the best resistance to crushing of a ply that has been embossed with a pattern with a high density.
In accordance with another feature, the product according to the invention has at least 30 protuberances per cm2 on at least 50% of the total surface.
In accordance with another feature the number of protuberances is at least 50 per cm2.
In accordance with another feature the product has a second embossed pattern between said portions of the surface. Advantageously, this pattern can be of a linear type, i.e. made up of protuberances whose surface at the top is in linear form.